This invention relates to an auditory hearing aid and more particularly to a hearing aid system and method which utilizes formant frequency transformation.
Although the conventional hearing aid, which simply amplifies speech signals, provides some relief from many hearing impairments suffered by people, there are many other types of hearing impairments for which the conventional hearing aid can provide little, if any, relief. In the latter situations, it is recognized that an approach different from simple amplification is necessary, and a number of different approaches have been proposed and tested at least in part. See Strong, W. J., "Speech Aids for the Profoundly/Severely Hearing Impaired: Requirements, Overview and Projections", The Volta Review, December, 1975, pages 536 through 556. Most of the methods and devices proposed to date, however, have proven unsatisfactory for either reception of speech by or training of hearing-impaired persons for whom the conventional hearing aid can provide no relief.
Many hearing-impared persons who cannot be helped by the conventional hearing aid nevertheless have residual hearing typically in a frequency range at the lower end of the frequency range of normal speech. Recognizing this fact, several different types of frequency-transposing aids have been suggested in which high-frequency energy of a speech signal is mapped or transposed into the low-frequency, residual hearing region. One of the frequency transposing methods produces arithmetic frequency shifts downward but in so doing may destroy information in the frequency range of the first format of the speech signal by replacing it with information from higher frequencies. Other methods compress the entire speech frequency range into the residual hearing range using vocoding techniques. If only a few frequency channels are used in the vocoding, the frequency resolution is too coarse to capture essential speech information. If many channels are used, too many frequencies are compressed into the narrow frequency band of residual hearing and they cannot be resolved. In both cases, it is likely that speech discrimination would suffer. In still other related methods, selected high frequency bands are mapped down into selected low frequency regions. Apparent drawbacks of these methods are the destruction of perceptually important low frequency information, the mapping of perceptually unimportant information, and the mapping of fixed frequency bands whether the speech is that of a male, female, or child.
Other speech reception aids which have been suggested include tactile aids, in which speech information is presented to the subject's sense of touch, and visual aids, in which speech information is visually presented to a subject. The obvious drawback of tactile and visual aids, as compared to auditory aids, is that the former occupy and require use of one of the person's senses which might otherwise be free to accomplish other tasks.